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7.6. What is the difference between a mora and a syllable?

Moras, counted as 1-paku, 2-haku, ... using haku (拍) or 1-on, 2-on, ... using on (音), are the rhythmic units of Japanese. A word combines moras to have a function in a phrase, and a morpheme is a combination of moras to have meaning in some context. The Japanese writing system of kana is based on moras, placing one kana on each mora, with some exceptions with 'softened consonants' like kya, known as yōon. See 7.7. What is yōon (youon)?

The difference between moras and syllables is:

• A long vowel is counted as 2 moras.
• Aa is one syllable, but two moras, a-a
• Ou is one syllable, but two moras, o-o
• Ei is one syllable, but two moras, e-e
• A syllabic n (ん) without a following vowel is counted as one mora, and these three are represented as a single n' mora (haneru oto). An is one syllable, but two moras, a-n'. Kankei "relationship" is two syllables, "kan" and "kei", but four moras, ka-n'-ke-e.
• Double consonants (sokuon) are counted as 1 mora (tsumaru oto). Thus, for example, oppai "breasts" is two syllables, "op"-"pai" but four moras, o-(p waiting)-pa-i, and ippai is two syllables, "ip"-"pai" but four moras, i-(p waiting)-pa-i.
• The Japanese phonetical system doesn't have any consonant teminated moras except n'. They do exist in practice, like the ending -des' for desu, but we consider it to have (neutral or half) vowel of /u/, and t', d' are considered to be /to/, /do/, respectively.

In changing English words to Japanese, for example 'trumpet' is pronounced torampetto. The moras are to-ra-n'-pe-(sokuon)-to, which makes six, although the original has two syllables, trum-pet. See also 5.1. How do I write an English word in Japanese?

There are also various reasons to consider Tookyoo "Tokyo" and hanbun "half" both to be two-syllable but four-mora words.

Acknowledgements

This answer is based on a newsgroup post by muchan.

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